In a dual turbine electric power plant, each of two steam turbines independently uses steam from a common steam supply to drive an associated electric generator. A dual turbine electric power plant offers an advantage that a malfunction of a turbine component or of its associated steam lines, control valves, and condensor may necessitate shutting down one, but not both of the turbine-generators, with the result that up to 50% of the total plant power output may be generated during such a shutdown. In a single turbine electric power plant, such a malfunction may necessitate shutting down the total generating capacity until necessary repairs are made.
When an electric power plant, whether single or dual turbine, includes a high temperature gas-cooled nuclear reactor in the steam supply, a superheat section of a steam generator supplies superheat steam to a high pressure portion of a turbine, while a reheat section of the steam generator furnishes reheat steam to a corresponding lower pressure portion of the turbine. Generally, the superheat steam improves the efficiency at which a turbine-generator converts steam energy to electrical energy. For purposes of protecting the steam generator from low steam flow through its superheat and reheat sections, a bypass line is connected across each turbine portion to permit a desired minimum steam flow through a section at times when the total flow of steam from that section through the turbine portions is less than the desired minimum.
In one type of power plant having a steam supply which includes a high temperature gas cooled nuclear reactor, auxiliary steam turbines are connected in the steam path which conducts steam, after its passage through the high pressure turbine portions and their associated bypass lines, to the inlet of the reheat section. The auxiliary steam turbines drive means for circulating the coolant gas through the reactor and the steam generator. At times when steam flows through the bypass lines associated with the lower pressure turbine portions, it is desirable to control the pressure of steam at the outlet of the reheater section, for such control improves control of the speed of the gas circulating means, and thus control of the flow rate of the coolant gas through the nuclear reactor.
An important aspect of the operation of a turbine-generator involves acceleration of the generator to synchronous speed, at which speed the generator may be connected to an associated power network. Prior to commencing such acceleration, the turbine-generator is rotated by a turning motor, typically at a few revolutions per minute, while the turbine portions are warmed by a gentle flow of steam. When acceleration commences, the turbine-generator is disconnected from the turning motor, and the shaft speed is controlled by governing the flow of steam through the turbine portions. Generally, the shaft speed is increased from turning gear speed to synchronous speed is increased from turning gear speed to synchronous speed at such a rate that no turbine part is subjected to harmful thermal conditions.
When a turbine-generator uses steam from a source that includes a high temperature gas-cooled nuclear reactor and auxiliary steam turbines are connected to drive the coolant gas circulators as above described, the presence of the auxiliary turbines necessitates that steam from the reheat section as well as the superheat section is at an elevated pressure when acceleration commences. Because of the elevated pressure of the steam emerging from the reheat section, acceleration of the turbine-generator cannot be controlled simply by fully opening the flow control valve to the inlet of the lower pressure portion while attempting to control the shaft speed by positioning the flow control valve to the inlet of the high pressure portion; rather, each of the steam flows through the high pressure turbine portion and the corresponding lower pressure portion must be controlled during acceleration. At the same time the pressure of steam at the outlet of the reheater section should be regulated according to its desired value.
In a proposed system for controlling the acceleration of a turbine-generator in a single turbine power plant wherein the steam supply includes a high temperature nuclear reactor cooled by a gas that is circulated by the above described auxiliary steam turbine, a controller for governing the pressure of steam emerging from the reheat section includes both proportional and integral modes. In a dual turbine plant, operation of two such controllers in concert may cause imbalances between the steam flows through the bypass lines associated with the lower pressure turbine portions. The proposed system permits a flow of steam through a lower pressure turbine portion by opening a two-position valve, which flow may vary, depending upon the flow characteristics of the valve and the turbine portion, which typically are temperature dependent. Such variation may affect the shaft speed, which is controlled by the steam flow through a high pressure turbine portion.
There appears to be a need for an acceleration control system for a turbine generator connected to a steam supply system such that each of first and second steam flows through portions of the turbine must be controlled during acceleration, the control system desirably varying only one of the steam flows for purposes of governing the shaft speed, while the other flow is held relatively constant. Such a control system is advantageously applicable to both single and dual turbine plants. Further desirably, such a system selects that one of the steam flows which is varied for speed control purposes according to the speed range within which the turbine-generator is operating. When more than one valve may be positioned to vary a steam flow for speed control purposes, that control valve which is used is selected to match advantageously the flow control characteristics of the valve to the range of speeds that it must govern. Especially in a power plant having a high temperature gas-cooled reactor in its steam supply system, the control system additionally desirably varies the flow of steam through the bypass line associated with the lower pressure turbine portion to regulate the pressure of steam at the outlet of the reheat section according to its desired value. Such pressure regulation improves the above-mentioned control of the speed of the coolant gas circulators, increases the accuracy of control of the shaft speed, and assures a desired minimum passage of steam through the reheat section.
The description of prior art herein is made on good faith and no representation is made that any prior art considered is the best pertaining prior art nor that the interpretation placed on it is unrebuttable.